Hi Olivier, and thanks for your questions!
Re: cloaking device and other items. Remember that any PC might have any item. At any time. The players get final say on what they want to achieve with their characters action; now, or in the past. And what specific action they think can pull that off.
That's one of the only rules of the game that should be held close (so much so, it's one of the few Core Rules). Anyways if that's rule one, then rule two is: the GM gets final say how much a description of a PC can achieve, given a particular goal and course of action. Rule 3 is that the GM decides whether a roll is needed and which type. And rule 4 is the players decide how many Karma they earn at the end of the session.
It's also up to the GM whatever else happens in the world as a result of a given action.
Given this, your questions are making it tough to answer. It's best not to set an edge cost for permission to take a particular action. You could set a cost for a flashback that's particularly outlandish, but it might be better to let them attempt that for free, and make a roll instead.
The additional questions that arise from trying to answer your questions indicate the need to discuss your experience and group in more detail. And quite frankly, the best answer for one situation for one group is almost never the best one for another situation, let alone another group. You sound like a conscientious player, with a desire to learn and impress your players! so I implore you to accept the Community Discord server invite included with your download, to better tap into the hive mind and get the help you deserve.
If that's Not possible, do respond here so I can better assist you
Cheers,
-Mark